Method for carbon nanotube purification

ABSTRACT

A method for carbon nanotube purification, preferably including: providing carbon nanotubes; depositing a mask; and/or selectively removing a portion of the mask; and optionally including removing a subset of the carbon nanotubes and/or removing the remaining mask.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/824,571, filed 19 Mar. 2020, which is a continuation of U.S.application Ser. No. 16/545,456, filed 20 Aug. 2019, which is acontinuation-in-part of prior U.S. application Ser. No. 16/191,185,filed on 14 Nov. 2018, all of which are incorporated in their entiretyby this reference.

TECHNICAL FIELD

This invention relates generally to the purification field, and morespecifically to a new and useful method for carbon nanotubepurification.

BACKGROUND

Typical methods for carbon nanotube purification tend to suffer from oneor more limitations. For example, such methods may be unable to preservethe alignment of carbon nanotubes in a sample being purified.Additionally or alternatively, such methods and systems may requiresurface patterning, use of high temperatures and/or energy expenditure,and/or may leave undesired residues on and/or around the purified carbonnanotubes.

Thus, there is a need in the purification field to create a new anduseful method for carbon nanotube purification.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a flowchart diagram of an embodiment of the method for carbonnanotube purification.

FIG. 2A is a schematic representation of an example of the method,including plan views of a carbon nanotube bed.

FIG. 2B is a schematic representation of an example of the method,including cross-sectional views of a carbon nanotube bed.

FIG. 2C is a schematic representation of the mask of FIG. 2B.

FIGS. 3A-3C are schematic representations of variations of elements ofthe method.

FIG. 4 is a schematic representation of an element of the method.

FIGS. 5A-5D are representations of optical absorption coefficients ofcarbon nanotubes in various wavelength ranges.

FIG. 6 is a schematic representation of a variation of elements of themethod.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments of the inventionis not intended to limit the invention to these preferred embodiments,but rather to enable any person skilled in the art to make and use thisinvention.

1. Overview

A method 100 for carbon nanotube (CNT) purification preferably includes:providing carbon nanotubes Silo; depositing a mask S120; and/orselectively removing a portion of the mask S130 (e.g., as shown in FIG.1). The method 100 can optionally include removing a subset of thecarbon nanotubes S140 and/or removing the remaining mask S150. However,the method 100 can additionally or alternatively include any othersuitable elements. The method 100 preferably functions to purify asample of CNTs, such as by isolating a desired population of the CNTs(e.g., by removing all or substantially all undesired CNTs, such asshown in FIG. 2A).

2. Benefits

The method 100 can confer several benefits. First, embodiments of themethod 100 can achieve highly-selective purification of CNTs (e.g.,resulting in CNTs with purity in excess of 99%, 99.9%, 99.99%, 99.999%,99.9999%, 99.99999%, or greater). Second, embodiments of the method 100can enable purification of CNTs (e.g., on arbitrary substrates, such asunpatterned substrates and/or substrates patterned for applicationsother than enabling the method 100) without perturbing their alignment(e.g., enabling preservation of the alignment of a bed of CNTs). Third,embodiments of the method 100 can be performed at low temperaturesand/or with low energy consumption, thereby enabling use oftemperature-sensitive substrates and/or reducing energy costs associatedwith CNT processing. Fourth, embodiments of the method 100 can enablehigh-throughput purification of CNTs, such as by performing the method100 using large area, broadband, and/or continuous-wave (CW)illumination. Fifth, embodiments of the method 100 can utilize clean,dry (e.g., liquid-free) processes that result in purified CNTs withminimal (e.g., substantially no) processing residues and/or othercontaminants. Sixth, embodiments of the method 100 can exclusivelyemploy materials and processes compatible with CMOS front-end of linerequirements. However, the method 100 can additionally or alternativelyconfer any other suitable benefits.

3. Method 3.1 Providing Carbon Nanotubes.

Providing carbon nanotubes S110 preferably functions to provide thematerial to be purified. The CNTs can include a mixture of two or moreCNT populations, wherein the method 100 preferably functions to isolateone or more such populations from the other(s). The populations caninclude (e.g., can be discriminated between based on) CNTs havingdifferent properties. These properties can include electrical and/orelectronic properties (e.g., based on bandgap, electrical resistance,properties arising from and/or associated with chirality, etc.) such asproperties along the tube axis, optical properties (e.g., absorptioncoefficient at one or more wavelengths, absorption spectrumcharacteristics, etc.), geometrical properties (e.g., diameter, length,chirality, orientation, etc.), thermal properties (e.g., thermalconduction properties, such as lateral, circumferential, and/or axialthermal conductivity, etc.), mechanical and/or acoustic properties(e.g., resonance modes), and/or any other suitable properties. In oneexample, the CNTs to be purified include two populations defined basedon CNT electrical properties (e.g., a metallic population and asemiconducting population), wherein the method 100 functions to removesubstantially all of the first CNT population, thereby purifying thesecond CNT population (e.g., remove the metallic population, resultingin substantially pure semiconducting CNTs). However, the providedmaterial can additionally or alternatively include any other suitablepopulations of CNTs.

The CNTs are preferably provided as a bed of CNTs, but can additionallyor alternatively be arranged in any other suitable manner. The CNTspreferably substantially form a monolayer (or partial monolayer),wherein the bed preferably includes minimal overlap of different CNTs.In particular, it may be desirable to minimize the number of CNTs to beremoved (e.g., CNTs of the first CNT population) that are arranged underone or more of the CNTs to be retained (e.g., CNTs of the second CNTpopulation), such as arranged between the CNT(s) to be retained and asubstrate, as such an arrangement may reduce the efficacy of the method100 (e.g., may result in the overlapped CNT of the first CNT populationnot being removed as intended, due to interference from the overlappingCNT of the second CNT population). However, it may additionally oralternatively be desirable to minimize the number of CNTs to be retained(e.g., CNTs of the second CNT population) that are arranged under one ormore of the CNTs to be removed (e.g., CNTs of the first CNT population),as such an arrangement may result in the overlapped CNT of the secondCNT population being damaged at and/or near the crossing (e.g., by theCNT removal process of S140). In some embodiments, the CNTs have acrossing density (e.g., number of overlapped CNT regions per unit area)less than a threshold amount (e.g., 1, 2, 5, 10, 15, 20, 25, 35, 50, 70,100, 200, 500, 1000, 0.1-1, 1-10, 10-20, 15-40, 40-100, 100-300,300-1000, or 1000-10,000 crossings per square micron, etc.), but canalternatively have any other suitable crossing density and/orarrangement of crossings.

The CNTs are preferably substantially aligned, such having substantiallyparallel longitudinal axes and/or terminating substantially along one ormore lines (e.g., reference lines defined by the CNT ends). In someembodiments, at least a threshold portion of the CNTs (e.g., 99.9, 99.5,99, 98, 95, 90, 80, 65, 50, 40-65, 65-85, 85-95, 95-99, or 99-100% ofthe CNTs, etc.) are arranged with their longitudinal axes within athreshold angle (e.g., 0.1, 0.5, 1, 2, 3, 5, 10, 15, 20, 0-1, 1-3, 3-10,10-30, or 30-45°, etc.) of a reference axis (e.g., longitudinalalignment axis). In one variation, the CNTs include multiple subsets(e.g., spatially separated subsets), wherein each subset issubstantially aligned along a different direction (e.g., longitudinalalignment axis). However, the CNTs can alternatively have any othersuitable arrangement.

The CNTs (e.g., the bed of CNTs) can have a low, medium, or highdensity, or have any other suitable density. In some embodiments (e.g.,in which the CNTs are provided as grown, in which a single batch ofgrown CNTs are transferred to another substrate, etc.), the density isless than and/or greater than a threshold linear density (e.g., 0.1,0.3, 1, 2, 5, 10, 20, 30, 0.001-0.1, 0.1-1, 1-3, 3-10, 10-30, 30-100, or100-300 CNTs per micron, etc.). In other embodiments (e.g., in whichmultiple batches of grown CNTs are transferred to a substrate), thedensity is less than and/or greater than a threshold linear density(e.g., 10, 20, 30, 50, 75, 100, 125, 150, 175, 200, 250, 300, 500, 1000,0.01-1, 1-10, 10-20, 15-30, 30-50, 50-80, 80-125, 125-175, 175-250,250-400, 400-600, 600-1000, or 1000-10,000 CNTs per micron, etc.). In afirst specific example, the linear density is at least 200 CNTs permicron (e.g., between 200 and 500 CNTs per micron). In a second specificexample, the linear density is at least 500 CNTs per micron. However,the CNTs can alternatively have any other suitable density. The lineardensity is preferably determined with respect to a direction (e.g.,within the plane of the substrate) substantially normal to thelongitudinal alignment axis, but can additionally or alternatively bedetermined with respect to any other suitable direction.

The CNTs are preferably single-walled CNTs (SWNTs), but can additionallyor alternatively include multi-walled CNTs (MWNTs) and/or any othersuitable CNTs. The CNTs preferably have substantially uniform diameters,such as wherein at least a threshold portion of the CNTs (e.g., 99.9,99.5, 99, 98, 95, 90, 80, 65, 50, 40-65, 65-85, 85-95, 95-99, or 99-100%of the CNTs, etc.) have diameters within a threshold absolute width(e.g., 0.1, 0.2, 0.5, 1, 2, 5, 10, 30, 100, 0.1-0.3, 0.3-1, 1-3, or 3-10nm, etc.) and/or relative amount (e.g., 0.1, 1, 2, 5, 10, 20, 50, 0-0.1,0.1-1, 1-2, 2-5, 5-10, 10-20, or 20-50%, etc.) of each other (e.g.,difference relative to a mean or median diameter, difference between anytwo CNTs, etc.), but can alternatively have substantially differentdiameters and/or any other suitable diameters. The CNT diameters, suchas the mean, median, minimum, maximum, first quartile, and/or thirdquartile diameters, can be 0.3, 0.43, 0.75, 0.9, 1, 1.1, 1.2, 1.3, 1.4,1.5, 1.6, 1.75, 2, 3, 5, 10, 0.43-0.75, 0.75-1.25, 1-1.5, 1.25-1.75,1.75-2.5, 2.5-5, 5-10, or 10-30 nm, and/or can be any other suitablediameters. In a specific example, the median diameter is in the range0.8-1.6 nm (e.g., approximately 1 nm, approximately 1.2 nm,approximately 1.5 nm, etc.).

The CNTs preferably have substantially uniform lengths, such as whereinthe lengths of at least a threshold portion of the CNTs (e.g., 99.9,99.5, 99, 98, 95, 90, 80, 65, 50, 40-65, 65-85, 85-95, 95-99, or 99-100%of the CNTs, etc.) differ by no more than a threshold absolute length(e.g., 1, 10, 100, 200, 500, 1000, 2000, 5000, 10,000, 50,000, 0.1-1,1-10, 10-100, 100-1000, 1000-10,000, or 10,000-100,000 nm, etc.) and/orrelative amount (e.g., 0.1, 1, 2, 5, 10, 20, 0-0.1, 0.1-1, 1-2, 2-5,5-10, or 10-20%, etc.) from each other and/or from a mean or medianlength, but can alternatively have substantially different lengthsand/or any other suitable lengths. The CNT lengths, such as the mean,median, minimum, maximum, first quartile, and/or third quartile lengths,can be 0.01, 0.1, 0.2, 0.5, 1, 2, 5, 10, 30, 100, 300, 1000, 0.001-0.1,0.1-1, 1-10, 10-100, 100-1000, or 1000-10,000 microns, and/or can be anyother suitable lengths.

The CNTs (e.g., bed of CNTs) are preferably supported by a substrate(e.g., arbitrary substrate), which can function to provide mechanicalsupport for the CNTs and/or facilitate handling and/or processing of theCNTs. The substrate can be a substrate on which the CNTs were grown,onto which the CNTs were transferred, a combination thereof (e.g.,wherein a first batch of CNTs are grown on the substrate, and one ormore additional batches of CNTs are transferred onto the substrate),and/or any other suitable substrate.

In some embodiments (e.g., in which S120 includes spin coating todeposit the mask), the substrate is preferably substantially smooth(e.g., wherein one or more roughness parameters, such as R_(a), R_(q),R_(v), R_(p), R_(t), R_(sk), R_(ku), R_(zDIN), R_(zJIS), S_(a), S_(q),S_(z), etc., is less than a threshold amount, such as 1, 10, 100, 1000,0-1, 1-10, 10-100, 100-1000, 1000-10,000, or 10,000-100,000 nmroughness, etc.). The substrate is preferably capable of tolerating thetemperatures (e.g., 100, 120, 140, 160, 180, 200, 250, 300, 20-60,60-100, 100-120, 120-140, 140-160, 160-180, 180-200, 200-250, 250-300,or 300-400° C., etc.) used in subsequent elements of the method 100(e.g., in S130 and/or S150, etc.), such as with substantially no damageto the substrate, but can alternatively not be tolerant of suchtemperatures. The substrate is preferably not susceptible to the inputsapplied in S130 (e.g., not substantially heated directly by the inputs,such as being substantially transparent to the light used to selectivelyheat the CNTs; being semi-transparent to the light, such as having 0,0.5, 1, 2, 5, 10, 0-1, 1-5, or 5-20% opacity; being highly reflective ofthe light, such as wherein the light is incident on the substrate-maskinterface at an angle shallower than the interface's critical angle;etc.). In one example, the substrate substantially transparent to theillumination used in S130 and is tolerant of temperatures up to at least160° C. However, the substrate can additionally or alternatively haveany other suitable properties.

The substrate can be single-crystal, poly-crystalline,micro-crystalline, nano-crystalline, amorphous, and/or can have anyother suitable microstructure. The substrate can be a wafer (e.g.,semiconductor wafer; wafer with a diameter such as 25 mm, 51 mm, 76 mm,100 mm, 125 mm, 130 mm, 150 mm, 175 mm, 200 mm, 300 mm, 450 mm, etc.),sheet (e.g., glass sheet, such as a sheet of the size corresponding toGEN 1, GEN 2, GEN 3, GEN 3.5, GEN 4, GEN 4.5, GEN 5, GEN 6, GEN 7, GEN7.5, GEN 8, GEN 9, GEN 10, GEN 10.5, etc.), block, and/or have any othersuitable shape. Example substrate materials can include quartz, silicon(e.g., bare, with one or more insulating layers such as an oxide and/ornitride layer, etc.), sapphire, glass (e.g., borosilicate glass, floatglass, etc.), plastic, fabric, and/or any other suitable materials. Thesubstrate can be bare (e.g., substantially uniform), patterned (e.g.,pre-patterned wafer, finished device (or finished except for the CNTs)such as a display, phone, etc., and/or an element thereof; etc.), and/orhave any other suitable features. However, the method 100 canadditionally or alternatively include providing the CNTs on any othersuitable substrate (and/or without any substrate).

The CNTs and/or substrate preferably include substantially noCMOS-incompatible materials. In a first example, the CNTs and/orsubstrate include substantially no metals (e.g., no trace contaminationby metal, no metal except trace contaminants, etc.). In a secondexample, the CNTs and/or substrate include substantially no metalsexcept for one or more CMOS-compatible metals (e.g., Al, W, Ti, Ni, Co,Pt, Al, Hf, Ta, Mo, W, Ti, Cr, Zr, Pd, etc.). However, the CNTs and/orsubstrate can additionally or alternatively include any other suitablematerials and/or contaminants.

S110 can optionally include growing (all or some of) the CNTs (e.g., onthe substrate; on a different substrate, such as prior to transfer tothe substrate; etc.). For example, S110 can include performing apatterned growth technique (e.g., as described in U.S. Pat. No.8,367,035, titled “Methods of making spatially aligned nanotubes andnanotube arrays”, which is hereby incorporated in its entirety by thisreference), preferably to grow an aligned bed of CNTs. S110 canadditionally or alternatively include transferring (all or some of) theCNTs to the substrate, such as by ink jet printing, thermal transferprinting, contact printing, dry transfer printing, screen printing,and/or any other suitable transfer technique (e.g., as described in U.S.Pat. No. 9,748,421, titled “Multiple carbon nanotube transfer and itsapplications for making high-performance carbon nanotube field-effecttransistor (CNFET), transparent electrodes, and three-dimensionalintegration of CNFETS”, which is hereby incorporated in its entirety bythis reference). However, S110 can additionally or alternatively includeproviding the CNTs in any other suitable manner.

3.2 Depositing a Mask.

Depositing a mask S120 preferably functions to protect and/orencapsulate the CNTs with a patternable material. The mask can bedeposited, for example, by spin coating, spray coating, dip coating,evaporation, sputtering, atomic layer deposition, self-assembly, and/orany other suitable deposition technique(s).

The mask preferably protects and/or encapsulates (e.g., covers)substantially all of the CNTs (e.g., substantially covers the entire bedof CNTs), but can alternatively protect and/or encapsulate any suitablesubset thereof. The mask preferably protects and/or encapsulates CNTs ofthe different populations (e.g., first and second population, such asdescribed below in more detail) to substantially the same extent, suchas wherein a similar (e.g., substantially equal) fraction of eachpopulation is protected and/or encapsulated (e.g., covered) by the mask.In embodiments in which the mask includes different portions protectingand/or encapsulating different CNT populations (e.g., a first portioncoating the first population and not the second population, and a secondportion coating the second population and not the first population), thedifferent portions of the mask are preferably defined by their spatialrelationships with the various populations (e.g., each portion isdefined as the subset of the mask that covers the corresponding CNTpopulation, such as shown by way of example in FIG. 2C), rather than byany other characteristics. The different portions preferably havesubstantially the same characteristics aside from their spatialrelationships with the various populations (e.g., the differentportions, preferably along with the rest of the mask, cooperatively forma substantially uniform, substantially continuous layer). For example,the different portions preferably include the same material(s) depositedby the same deposition process and are preferably deposited at the sametime (e.g., the first and second portions are deposited concurrently inthe same deposition process). The mask preferably does not include anylateral patterning corresponding to differing locations of CNTs of thedifferent populations. In some such examples, in the absence of theCNTs, it would not be possible to discriminate between differentportions of the mask (e.g., it would not be possible to determine theboundaries of any particular portion of the mask, nor to determine, forany particular location on the mask, which portion that location isassociated with).

The mask (e.g., after deposition) is preferably made substantially of asingle material (e.g., at least 75% purity, 90% purity, 95% purity, 99%purity, 99.9% purity, etc.; including or excluding one or moresubstances, such as solvents, associated with mask deposition), but canalternatively include multiple materials. The mask material ispreferably operable to be deposited (e.g., easily deposited) in a thin,uniform layer, but can additionally or alternatively have any othersuitable deposition characteristics. The material is preferably apolymer, but can additionally or alternatively include small organicmolecules, metals, semiconductors, ceramics, and/or any other suitablematerials. The mask is preferably not susceptible to the inputs appliedin S130 (e.g., not substantially heated directly by the inputs, such asbeing substantially transparent to the light used to selectively heatthe CNTs).

The mask (e.g., all or some of the mask) is preferably in thermalcontact with the CNTs. For example, the mask can form a layer (e.g.,coating, such as a conformal coating) on and between the CNTs (e.g., onthe exposed substrate between the CNTs). The mask is preferablysubstantially uniform (e.g., uniform thickness, such as uniformconformal coating thickness or uniform distance from the substrate tothe top of the mask layer; uniform composition, such as laterallyuniform across the substrate or a portion thereof; etc.) but canalternatively have any suitable nonuniformities. The mask is preferablythick enough to protect the CNTs that it covers (e.g., during S140), butpreferably thin enough to enable formation (e.g., during S130) of thintrenches (e.g., less than a threshold width, such as 1, 2, 5, 8, 10, 12,15, 20, 0-1, 1-5, 5-15, 15-20, or 20-35 nm wide), which can enable highselectivity (e.g., avoiding inadvertent exposure of CNTs of the wrongpopulation) and/or reduce the energy needed to perform the method 100(e.g., to perform S130 and/or S150). For example, the mask can bethinner than a first threshold thickness (e.g., 2, 5, 8, 10, 12, 15, 20,30, 50, 1-5, 5-15, 15-20, 20-30, 30-50, or 50-100 nm) and/or thickerthan a second threshold thickness (e.g., 0.5, 1, 2, 5, 8, 10, 12, 15,20, 0-1, 1-5, 5-15, 15-20, or 20-35 nm). However, the mask canadditionally or alternatively have any other suitable thickness.

The mask is preferably susceptible to decomposition (e.g.,depolymerization) in response to one or more stimuli (e.g., includes oneor more stimuli-responsive, depolymerizable, low ceiling temperaturepolymers). The stimulus is preferably a heat-related stimulus such as atemperature increase (e.g., increase to above a threshold temperature,such as 30, 40, 60, 80, 100, 120, 140, 160, 180, 200, 250, 300, 20-60,60-100, 100-120, 120-140, 140-160, 160-180, 180-200, 200-250, 250-300,or 300-400° C., etc.), but can additionally or alternatively be anyother suitable stimulus. The material decomposition process ispreferably a clean process (e.g., leaves minimal or substantially noresidue), such as a process that does not require subsequent removal ofmaterial residue (e.g., by etching, washing with one or more solvents,etc.). For example, the decomposition process can be a depolymerizationprocess, producing monomers (e.g., volatile monomers) that leave theCNTs (e.g., vaporize, are pumped away, etc.). However, the material canadditionally or alternatively have any other suitable characteristics.

A person of skill in the art will recognize that depolymerization is theprocess of decomposing a polymer into its constituent monomers and/orconstituent oligomers. The products of depolymerization can be mostly(e.g., all, substantially all such as more than 90%, 95%, 98%, 99%,99.9%, 99.99%, etc.) the constituent monomers, but can additionally oralternatively include constituent oligomers (e.g., include a mixture ofmonomers and oligomers, include substantially only oligomers, etc.). Theconstituent oligomers can include oligomers with any suitable number ofrepeat units (e.g., 2, 3, 4, 5, 6-10, 10-20, and/or 20-50 repeat units,etc.). The products of depolymerization preferably consist essentiallyof the constituent monomers and/or constituent oligomers. Theconstituent monomers produced by depolymerization are preferably themonomers associated with the repeat units and/or structural units of thepolymer that was depolymerized (e.g., the monomers are molecular analogsof the repeat units and/or structural units). In some examples, in whichthe polymer includes multiple structural units of different types (e.g.,in a polymer, such as a condensation polymer, formed from a plurality ofmonomer species), depolymerization can produce multiple types ofconstituent monomers (e.g., associated with the structural units, suchas being the plurality of monomer species from which the polymer wasformed or a subset thereof) and/or constituent oligomers, or can producea single type of constituent monomer (e.g., associated with the repeatunits) and/or constituent oligomer. In some examples, depolymerizationincludes an unzipping process (“unzipping depolymerization”), in whichthe depolymerization occurs by a sequence of reactions progressing alonga polymer (e.g., beginning from one end of the polymer, progressingmonomer-by-monomer, etc.) to yield constituent monomers (and/orconstituent oligomers). A person of skill in the art will furtherrecognize that depolymerization is an example of polymer decomposition,and that polymers can undergo many other decomposition processes (e.g.,pyrolysis, gasification, solvolysis, etc.), all of which are distinctfrom depolymerization. For example, in contrast with depolymerization,pyrolysis typically produces an assortment of small molecules (notlimited to the constituent monomers and/or constituent oligomers of thepolymer) and carbonaceous residue.

In some embodiments, the material is (or includes) a stimulus-responsivepolymer, which is a polymer configured to decompose, preferably bydepolymerization, in response to a trigger event. Such polymers caninclude self-immolative polymers, which are polymer chains stabilized byone or more trigger groups (e.g., end caps). In the absence of thetrigger group(s), the polymer chain decomposes (e.g., depolymerizes,preferably via unzipping depolymerization beginning from the end atwhich the trigger group would be present in order to stabilize thepolymer) when above a threshold temperature, typically a low temperature(e.g., in the range of −60° C. to 0° C., such as −20° C. to −40° C.).The trigger group(s) can function to prevent this decomposition whenthey are bound to the polymer chain (e.g., at one or more ends of thechain); upon loss of the trigger group, such as due to trigger groupcleavage and/or decomposition (e.g., in response to the stimulus, suchas in response to reaching a temperature above a threshold such as100-180° C.), the polymer chain preferably undergoes decomposition(e.g., depolymerization, preferably unzipping depolymerization), such asdescribed above. In one example, in which the material includes aself-immolative polymer, the trigger group is an end cap such astrichloroacetyl isocyanate (TCAI), azobenzene, chloroformate, or aceticanhydride.

The self-immolative polymers (e.g., with end cap(s) and/or other triggergroup(s), such as described above) can include, for example,poly(N-isopropyl acrylamide), poly(N-ethylpyrrolidine methacrylate),poly(phthalaldehyde), poly(4,5-dichlorophthalaldehyde), poly(methylglyoxylate), poly(carbamate), polyurethane, polycarbonate, and/orpoly(benzyl ether), or a combination of multiple such materials. Thepolymer chain can be branched, linear, cyclic, and/or have any othersuitable configuration. The polymer chain can be between 10-1,000 nmlong, or have any suitable length. The polymers units in the maskpreferably have similar and/or substantially identical properties, butcan alternatively be variable. However, the mask can additionally oralternatively include any other suitable polymers and/or othermaterials.

The mask preferably includes substantially no CMOS-incompatiblematerials. In a first example, the mask includes substantially no metals(e.g., no trace contamination by metal, no metal except tracecontaminants, etc.). In a second example, the mask includessubstantially no metals except for one or more CMOS-compatible metals(e.g., Al, W, Ti, Ni, Co, Pt, Al, Hf, Ta, Mo, W, Ti, Cr, Zr, Pd, etc.).However, the mask can additionally or alternatively include any othersuitable materials and/or contaminants.

However, the mask can additionally or alternatively include any othersuitable material(s), have any other suitable characteristics, and/orcan be deposited in any other suitable manner.

3.3 Selectively Removing a Portion of the Mask.

Selectively removing a portion of the mask S130 preferably functions topattern the mask (e.g., based on proximity to CNTs of one or morepopulations, thereby enabling discrimination between the CNTpopulations). The mask portion is preferably removed by decomposition(e.g., depolymerization), such as described above (e.g., regardingS120), but can additionally or alternatively be removed in any othersuitable manner. The removed mask portion preferably leavessubstantially no residue and/or other contamination (e.g., on the CNTs,on the substrate, etc.). For example, the decomposition products (e.g.,volatile monomers formed by depolymerization) can be pumped away fromthe CNTs. S130 can be performed in an ambient environment (e.g., air),inert environment (e.g., nitrogen, argon, etc.), vacuum environment(e.g., partial vacuum such as negative pressure environment, roughvacuum, high vacuum, ultra high vacuum, etc.), and/or any other suitableenvironment.

S130 preferably includes removing the mask portion (“first maskportion”) covering (e.g., coating) the first CNT population (e.g.,population to be removed), and preferably includes not removing (and/orminimizing the removal of) the mask portion (“second mask portion”)covering (e.g., coating) the second CNT population (e.g., population tobe retained). However, S130 can additionally or alternatively includeremoving the mask portion covering any other CNT population(s), and/orremoving any other suitable portions of the mask.

Removing the first mask portion preferably exposes all or some of eachCNT of the first CNT population (or a substantial fraction thereof, suchas at least 90, 99, 99.9, 99.99, 99.999, 99.9999, or 99.99999% of theCNTs of the first CNT population), such as exposing at least a minimumthreshold fraction and/or at most a maximum threshold fraction of thetube width of each such CNT (e.g., 1, 2, 5, 10, 20, 30, 40, 50, 60, 70,80, 90, 95, 98, 99, 100, 0-1, 1-10, 10-30, 30-60, 60-80, 80-90, 90-95,95-99, or 99-100% of the tube width) and/or exposing a region extendingpast the tube width on one or both sides (e.g., extending by at least aminimum threshold fraction and/or at most a maximum threshold fraction,such as 1, 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 95, 98, 99, 100,0-1, 1-10, 10-30, 30-60, 60-80, 80-90, 90-95, 95-99, or 99-100% of thetube width). In some embodiments, S130 includes opening a trench overand/or around each CNT of the first CNT population (or substantially allsuch CNTs); example shown in FIG. 2B. Each trench preferably has a width(e.g., mean width, median width, minimum width, maximum width, etc.,along its length or a portion thereof) such as described above (e.g.,regarding the minimum and/or maximum threshold fractions of tube widthto expose; regarding S120, such as regarding the mask thickness; etc.).In one example (e.g., in which all or some of the CNTs have widths inthe range of 1-1.5 nm), all or some of the trenches have widths of0.5-20 nm, preferably 1-10 nm (e.g., 1, 2, 3, 5, 7.5, 10, 1-2, 2-5, or5-10 nm). However, the trenches can additionally or alternatively haveany other suitable widths.

Selectively removing a portion of the mask S130 preferably does notresult in (or minimizes the occurrence of) one or more undesired maskconfigurations. A first example of an undesired mask configurationincludes opening a trench extending excessively past the width of afirst population CNT (e.g., extending significantly more than necessaryto enable S140), such that the trench also exposes one or more secondpopulation CNTs, which can result in damage to and/or removal of theexposed second population CNTs (e.g., as shown in FIG. 3A). A secondexample of an undesired mask configuration includes exposing (e.g.,opening a trench over) a second population CNT (e.g., rather than and/orin addition to over a first population CNT), which can result in damageto and/or removal of the exposed second population CNT (e.g., as shownin FIG. 3B). A third example of an undesired mask configuration includesfailing to expose (e.g., open a trench over) a first population CNT,which can result in failure to remove the unexposed first population CNT(e.g., as shown in FIGS. 3B-3C). However, S130 can additionally oralternatively include avoiding and/or minimizing any other suitableundesired mask configurations, or can include no such avoidance and/orminimization.

S130 preferably includes selectively heating one or more target CNTpopulations S131 (e.g., preferentially heating the first CNT population,such as heating the first CNT population more than the second CNTpopulation). S131 preferably results in heat transmission to regions ofthe mask near the target CNTs (e.g., to the first mask portion), therebyelevating the temperature of the heated regions above the mask'sdecomposition temperature (e.g., above the temperature at which thetrigger groups no longer stabilize the stimulus-responsive polymer ofthe mask, thereby causing depolymerization) and causing those regions todecompose. However, S131 preferably does not result in significant heattransmission (e.g., sufficient to cause mask decomposition) to regionsof the mask near untargeted CNTs (e.g., due to undesired heating of theuntargeted CNTs, due to heat transmission from the target CNTs, etc.),thereby avoiding decomposition of those regions (e.g., the second maskportion). Thus, S131 preferably includes a substantial difference in theheating rate between the target CNT populations and the untargeted CNTs(e.g., more than a threshold ratio, such as 1.1, 1.5, 2, 3, 5, 10, 20,1-1.2, 1.2-2, 2-5, 5-10, 10-20, or 20-50 times greater heating of thetarget CNT populations).

S131 preferably includes heating the target CNT populations optically(e.g., due absorption of an optical input), more preferably byilluminating the CNTs with light that is preferentially absorbed by thetarget CNT populations (e.g., absorbed more strongly, preferablysignificantly more strongly, by first population CNTs than by secondpopulation CNTs), such as shown in FIG. 4. This preferential absorptionpreferably arises from a difference in absorption coefficients(associated with the light of the optical input) between CNTs of thefirst and second populations (e.g., wherein CNTs of the first populationhave a greater absorption coefficient than CNTs of the secondpopulation), but can additionally or alternatively arise from any othersuitable phenomena.

In one embodiment, in which the populations include a metallic CNTpopulation and a semiconducting CNT population, the S131 includesilluminating the CNTs using one or more wavelength bands that arepreferentially absorbed by either metallic or semiconducting CNTs (e.g.,wavelengths such as shown in FIGS. 5A-5D and/or as described in“Absorption spectra of high purity metallic and semiconductingsingle-walled carbon nanotube thin films in a wide energy region”, MasaoIchida et al., Solid State Communications, 151 (2011) 1696-1699, whichis hereby incorporated in its entirety by this reference). In a firstvariation, in which the first CNT population (population to be heated)is the metallic CNT population, the illumination can include one or morebands in the IR range 2-4 microns (e.g., in the range 3-4 microns), thevisible range 500-800 nm (e.g., in the range 600-750 nm), in thevisible/UV range 300-450 nm (e.g., in the range 365-425 nm), in one ormore THz ranges (e.g., 60 microns or longer, such as in the range60-1250 microns), and/or any other suitable bands. In a secondvariation, in which the first CNT population is the semiconducting CNTpopulation, the illumination can include one or more bands in the IRrange 0.8-2 microns (e.g., in the range 0.9-1.9 microns) and/or anyother suitable bands.

The illumination preferably includes one or more wide bands (e.g., bandshaving a wavelength bandwidth greater than a threshold value, such as 5,10, 20, 50, 100, 200, 500, 750, 1000, 1500, 2000, 1-5, 5-25, 25-100,100-300, 300-1000, 1000-3000, or 3000-30,000 nm, etc.), but canadditionally or alternatively include narrow bands (e.g., bandwidth lessthan the threshold value, substantially monochromatic, etc.) and/or anyother suitable illumination.

The illumination can be incident on the CNTs from a direction parallel,normal, and/or at an oblique angle (e.g., glancing, such as within 5° ofparallel; slightly off-normal, such as within 5° of normal;substantially oblique, such as 10, 15, 30, 45, 60, 75, 80, 5-10, 10-20,20-40, 40-50, 50-70, 70-80, or 80-85°, etc.) to the substrate and/or thelongitudinal alignment axis.

The illumination preferably has an intensity in the range of milliwattsper square centimeter to watts per square centimeter (e.g., 1-3, 3-10,10-20, 20-50, 50-100, 100-300, 300-1000, 1000-3000, or 3000-30,000mW/cm², etc.), but can alternatively have a lower or higher intensity.In one example, the illumination intensity is in the range 10-30 mW/cm².The illumination is preferably continuous wave (CW) illumination, butcan additionally or alternatively include pulsed illumination and/or anyother suitable temporal variation in illumination. The CNT bed can beilluminated for a time period of seconds (e.g., 1, 2, 5, 10, 25, 60,0.5-3, 3-20, or 20-60 s), minutes (e.g., 1, 2, 3, 5, 10, 1-3, or 3-10minutes), or any other suitable length of time.

The illumination preferably originates from a broadband light source(e.g., emitting light due to thermal radiation) such as a lamp. Forexample, a system for illuminating the CNT bed can include a broadbandlight source, one or more optical elements (e.g., lenses, mirrors, etc.)that can function to focus light from the source onto the CNT bed,and/or one or more optical filters that can function to select theappropriate band(s) of light for illumination (e.g., as describedabove). In this example, some variations of the system can be easilyreconfigured for different illumination conditions (e.g., usingdifferent wavelength bands, such as to target different CNTpopulations), such as by replacing one or more of the filters. However,the illumination can additionally or alternatively originate from one ormore narrowband and/or monochromatic light sources, such as LEDs and/orlasers (e.g., bar diode, point source, etc.), and/or from any othersuitable light sources.

S131 can additionally or alternatively include heating the target CNTpopulations using one or more inputs such as electrical inputs (e.g.,DC, AC), radio wave inputs (e.g., microwave), acoustic inputs, and/ormechanical inputs. In a first example, the CNT bed is supported by asubstrate that includes electrical contacts, such as a first electricalcontact in contact with a first end of CNTs of an aligned array, and asecond electrical contact in contact with the opposing end of thoseCNTs. In this example, an electrical bias is applied between the twoelectrical contacts, thereby causing current to flow through thecontacted CNTs, wherein significantly more current flows through eachmetallic CNT than through each semiconducting CNT, thereby causingresistive heating of the metallic CNTs. In a variation of this example,a bias is also applied to a gate electrode (e.g., patterned on thesubstrate, opposing the substrate across the CNT bed, such as on top ofand/or integrated with the mask, etc.), which is preferably capacitivelycoupled to the CNT bed (e.g., to the semiconducting CNTs of the bed),thereby reducing and/or preventing current flow through thesemiconducting CNTs. In a second example, the CNT bed is supported by asubstrate that includes one or more antennas (e.g., microwave antennas)that can function to couple incident radiation (e.g., radio waves, suchas microwaves) into the CNT bed, thereby inducing a current in the CNTs(e.g., preferentially in the metallic CNTs). In this example, radiationresonant with the antenna(s) is transmitted toward the antennas, therebyinducing current in CNTs, causing preferential heating of the metallicCNTs (e.g., as described above regarding the first example). In a thirdexample, an acoustic and/or mechanical input is supplied to the CNT bed,wherein the input includes one or more frequencies that are resonantwith CNTs of the first population (e.g., wherein the populations aredetermined based on acoustic and/or mechanical properties), thus causingthe first population CNTs to preferentially absorb energy from theinput, which is subsequently dissipated as heat.

The energy input (e.g., illumination) is preferably incident on a largearea (e.g., the entire CNT bed; a portion thereof, such as more than 1,2, 5, 10, 20, 25, 30, or 50%; etc.) at once, but can alternatively beincident on a region of any suitable size. In a first example, theenergy input is incident on the entire CNT bed, and S131 can beperformed by directing energy to (e.g., illuminating) the entire CNTbed, preferably until the desired temperature increase has been achievedin the first mask portion. In a second example, the energy input isincident on only a subset of the CNT bed, and S131 can be performed bymoving the energy input location and/or the CNT bed (e.g., rastering theenergy input to cover the entire CNT bed), such as by completing maskremoval in each region before moving to the next, or by rapidly movingbetween regions (e.g., to achieve concurrent mask removal in multiplesuch regions).

However, S131 can additionally or alternatively include using any othersuitable energy input in any suitable manner.

S130 can optionally include causing additional heating (e.g., uniformheating) of all the CNTs (e.g., across the entire CNT bed and/orsubstrate). For example, S130 can include heating the CNT bed (e.g.,using an oven, hot plate, and/or other heat source) to a substantiallyuniform temperature below the mask decomposition temperature (e.g., 1,2, 5, 10, 15, 20, 25, 0-3, 3-10, 10-20, or 20-50° C. below), butpreferably above the ambient temperature (e.g., 18-22° C.). Thus, theselective heating required in S131 can be significantly reduced (e.g.,requiring a mask temperature increase of 5-20° C. rather than 50-200°C.), which can reduce the needed power and/or dwell time associated withS131. In a specific example, in which the mask decomposition temperatureis approximately 140° C., S130 includes heating the substrate, CNT bed,and mask to approximately 130° C., and then performing S131 (e.g.,illuminating the CNT bed with light preferentially absorbed by the firstpopulation) to elevate the temperature of the first mask portion above140° C. while preventing the second mask portion from reaching orexceeding 140° C. (e.g., by continuing to maintain an overall or averagetemperature of approximately 130° C.).

However, S130 can additionally or alternatively include selectivelyremoving a portion of the mask in any other suitable manner.

3.4 Removing a Subset of the Carbon Nanotubes.

Removing a subset of the carbon nanotubes S140 preferably functions topurify the CNTs.

S140 preferably includes removing the exposed CNTs (e.g., first CNTpopulation, CNTs near the first mask portion, etc.). Removing theexposed CNTs preferably includes removing substantially all of theexposed CNTs (e.g., more than a threshold fraction, such as 99, 99.9,99.99, 99.999, 99.9999, or 99.99999%, etc.). Each removed CNT ispreferably substantially entirely vaporized (e.g., wherein the vaporizedmaterial is pumped away from the CNT bed), and preferably leavessubstantially no conductive residue and/or other residue on the CNT bed,mask, and/or substrate.

The exposed CNTs are preferably removed by one or more etchingprocesses. The etching process(es) preferably remove (e.g., destroy) theexposed CNTs without etching through the mask (e.g., without exposingthe previously-unexposed CNTs, such as the second CNT population). Insome embodiments, in which the mask layer thickness is significantlygreater than the CNT diameter (e.g., a 5-20 nm mask thickness comparedto a 1-1.5 nm CNT diameter), even substantially non-selective etchingprocesses and/or some etching processes that etch the mask more quicklythan the CNTs can be tolerated (e.g., as a small CNT will still bedestroyed before the etching process removes the entire mask thickness).S140 can optionally include etching partially-exposed CNTs. For example,if the trench width is less than the CNT diameter, the etch process can(and preferably does) undercut to remove the entire CNT (e.g., as shownin FIG. 6). The etching processes can include, for example, oxygenplasma etching, UV-ozone etching, physical etching (e.g., argon ionmilling), chemical etching (e.g., vapor-phase etching, wet etching,etc.), and/or any other suitable etching processes. However, the exposedCNTs can additionally or alternatively be removed by washing (e.g.,using mechanical forces from a washing fluid to remove the CNTs) and/orany other suitable removal processes.

S140 can additionally or alternatively include removing the unexposedCNTs (e.g., second CNT population, CNTs near the second mask portionand/or not near the first mask portion, etc.). For example, S140 caninclude delaminating the remaining mask from the substrate (e.g.,peeling the mask off the substrate), wherein the unexposed CNTs remainembedded in the remaining mask and are thereby removed from thesubstrate. These CNTs can, for example, subsequently be transferred to adifferent substrate and/or used in any other suitable manner.

However, S140 can additionally or alternatively include removing anysuitable carbon nanotubes in any suitable manner.

3.5 Removing the Remaining Mask.

Removing the remaining mask S150 preferably functions to expose thepurified CNTs (e.g., the CNTs still on the substrate, such as the secondCNT population). Such exposure can enable, for example: transferring thepurified CNTs (e.g., onto and/or into an electronic device), such astransferring multiple batches of purified CNTs (e.g., to achieve a highCNT density); device fabrication (e.g., incorporating the CNTs, such asfabricating on top of the purified CNT bed); packaging and/or use of afinished device (e.g., finished after addition of the CNTs); and/or anyother suitable use of the purified CNTs.

S150 preferably includes causing the remaining mask to decompose,analogous to the decomposition of portions of the mask in S130 (e.g.,using the same stimulus to cause depolymerization of thestimulus-responsive polymer). For example, S150 can include heating themask (e.g., by heating the entire substrate, such as on a hot plateand/or in an oven) to a temperature greater than its decompositiontemperature, thereby causing the mask to depolymerize. Such a removaltechnique can be beneficial for several reasons. For example, it canresult in little or substantially no residue from the mask remaining onthe CNTs and/or substrate, it can be performed as a dry (liquid-free)process, and/or can be easily performed at large scale (e.g.,concurrently for the entire substrate and/or multiple substrates).

S150 can additionally or alternatively include removing the remainingmask by dissolving the mask (e.g., in a chemical solvent), etching themask (e.g., using an dry and/or wet etching process that selectivelyetches the mask and not the CNTs), delaminating (e.g., peeling) the maskfrom the substrate (e.g., wherein the CNTs remain on the substrate,rather than being retained within the mask), and/or removing theremaining mask in any other suitable manner.

4. Specific Example

In one example, the method 100 is performed to purify a substantiallyaligned monolayer bed of CNTs supported on a substrate. The mask isdeposited S120 by coating the CNT bed with an approximately 10 nm thicklayer of one or more self-immolative polymers (e.g., by spin coating asolution of the polymer(s) dissolved in one or more solvents). Aftermask deposition, the entire CNT bed is illuminated with one or morewavelength bands (e.g., approximately 750 nm wide) within the 2-4 micronrange and/or the 600-750 nm range, thereby selectively heating themetallic CNTs more than the semiconducting CNTs, which causes trenches(e.g., approximately 10 nm wide trenches) to form in the mask above themetallic CNTs and not above the semiconducting CNTs (as the maskdepolymerizes in the vicinity of the heated metallic CNTs). Afterillumination, the exposed (metallic) CNTs are etched in an oxygenplasma. After exposed CNT etching, the remaining polymer mask is removedby heating the substrate above the decomposition temperature (e.g., fora 140° C. decomposition temperature, heating to approximately 200° C.),thereby exposing the purified semiconductor CNTs.

In a second example, the method 100 is performed to purify asubstantially aligned monolayer bed of CNTs supported on a substrate.The mask is deposited S120 by coating the CNT bed with an approximately10 nm thick layer of one or more self-immolative polymers (e.g., by spincoating a solution of the polymer(s) dissolved in one or more solvents).After mask deposition, the entire CNT bed is illuminated with one ormore wavelength bands (e.g., approximately 750 nm wide) within the 0.8-2micron range, thereby selectively heating the semiconducting CNTs morethan the metallic CNTs, which causes trenches (e.g., approximately 10 nmwide trenches) to form in the mask above the semiconducting CNTs and notabove the metallic CNTs (as the mask depolymerizes in the vicinity ofthe heated semiconducting CNTs). After illumination, the exposed(semiconducting) CNTs are etched in an oxygen plasma. After exposed CNTetching, the remaining polymer mask is removed by heating the substrateabove the decomposition temperature (e.g., for a 140° C. decompositiontemperature, heating to approximately 200° C.), thereby exposing thepurified metal CNTs.

However, the method 100 can additionally or alternatively include anyother suitable elements performed in any suitable manner.

Although omitted for conciseness, embodiments of the system and/ormethod can include every combination and permutation of the varioussystem components and the various method processes, wherein one or moreinstances of the method and/or processes described herein can beperformed asynchronously (e.g., sequentially), concurrently (e.g., inparallel), or in any other suitable order by and/or using one or moreinstances of the systems, elements, and/or entities described herein.

The FIGURES illustrate the architecture, functionality and operation ofpossible implementations of systems and/or methods according topreferred embodiments, example configurations, and variations thereof.It should also be noted that, in some alternative implementations, thefunctions noted in the block can occur out of the order noted in theFIGURES. For example, two blocks shown in succession may, in fact, beexecuted substantially concurrently, or the blocks may sometimes beexecuted in the reverse order, depending upon the functionalityinvolved. It will also be noted that each block of the block diagramsand/or flowchart illustration, and combinations of blocks in the blockdiagrams and/or flowchart illustration, can be implemented by specialpurpose hardware-based systems that perform the specified functions oracts, or combinations of special purpose hardware and computerinstructions.

As a person skilled in the art will recognize from the previous detaileddescription and from the figures and claims, modifications and changescan be made to the preferred embodiments of the invention withoutdeparting from the scope of this invention defined in the followingclaims.

We claim:
 1. A method for carbon nanotube purification, comprising:coating a bed of carbon nanotubes with a polymer mask, the bedcomprising a first population of carbon nanotubes and a secondpopulation of carbon nanotubes, wherein the polymer mask comprises: afirst portion coating the first population and not the secondpopulation; and a second portion coating the second population and notthe first population; selectively depolymerizing the first portion ofthe polymer mask and not the second portion of the polymer mask,comprising illuminating the bed of carbon nanotubes with light, whereinthe light is more strongly absorbed by the first population than by thesecond population, thereby heating the first portion of the polymermask; after depolymerizing the first portion of the polymer mask,selectively removing the first population of carbon nanotubes and notthe second population of carbon nanotubes, wherein the second portion ofthe polymer mask protects the second population during removal of thefirst population; and after selectively removing the first population ofcarbon nanotubes, removing the second portion of the polymer mask fromthe bed of carbon nanotubes.
 2. The method of claim 1, wherein: thefirst population consists essentially of metallic carbon nanotubes; andthe second population consists essentially of semiconducting carbonnanotubes.
 3. The method of claim 2, wherein, after selectively removingthe first population of carbon nanotubes, a metallic/semiconductingratio of the bed of carbon nanotubes is less than 0.001.
 4. The methodof claim 1, wherein the carbon nanotubes of the bed are arrangedsubstantially in a single monolayer or partial monolayer.
 5. The methodof claim 4, wherein substantially all carbon nanotubes of the bed arearranged substantially parallel to each other.
 6. The method of claim 1,wherein the polymer mask comprises at least one of: poly(N-isopropylacrylamide), poly(N-ethylpyrrolidine methacrylate),poly(phthalaldehyde), poly(4,5-dichlorophthalaldehyde), poly(methylglyoxylate), poly(carbamate), polyurethane, polycarbonate, orpoly(benzyl ether).
 7. The method of claim 1, wherein a thickness of thepolymer mask is less than 30 nm.
 8. The method of claim 1, whereinilluminating the bed of carbon nanotubes with light comprises:illuminating the carbon nanotubes with light defining a wavelengthbandwidth of more than 25 nm.
 9. The method of claim 1, wherein removingthe first population of carbon nanotubes comprises exposing the firstpopulation of carbon nanotubes and the second portion of the polymermask to an etchant environment.
 10. The method of claim 1, wherein thebed of carbon nanotubes is supported by a substrate, wherein thesubstrate is substantially transparent to the light.
 11. The method ofclaim 1, wherein: the first population consists essentially ofsemiconducting carbon nanotubes; and the second population consistsessentially of metallic carbon nanotubes.
 12. A method for carbonnanotube purification, comprising: coating a substantially aligned bedof carbon nanotubes with a polymer mask, the bed comprisingsemiconducting carbon nanotubes and metallic carbon nanotubes, whereinthe polymer mask comprises: a first portion coating the metallic carbonnanotubes and not the semiconducting carbon nanotubes; and a secondportion coating the semiconducting carbon nanotubes and not the metalliccarbon nanotubes; selectively depolymerizing the first portion of thepolymer mask and not the second portion of the polymer mask, comprisingselectively heating the metallic carbon nanotubes more than thesemiconducting carbon nanotubes, thereby heating the first portion ofthe polymer mask; and after depolymerizing the first portion of thepolymer mask, selectively etching the metallic carbon nanotubes and notthe semiconducting carbon nanotubes, wherein the second portion of thepolymer mask protects the semiconducting carbon nanotubes duringetching.
 13. The method of claim 12, wherein selectively heating themetallic carbon nanotubes and not by the semiconducting carbon nanotubescomprises: illuminating the carbon nanotubes with light, wherein thelight is more strongly absorbed by the first population than by thesecond population.
 14. The method of claim 13, wherein illuminating thebed of carbon nanotubes with light comprises: illuminating the carbonnanotubes with light emitted by a broadband light source.
 15. The methodof claim 13, wherein illuminating the bed of carbon nanotubes with lightcomprises: illuminating the carbon nanotubes with light defining awavelength between 600 nanometers and 750 nanometers.
 16. The method ofclaim 12, wherein selectively heating the metallic carbon nanotubes andnot by the semiconducting carbon nanotubes comprises: applying a voltagebias across the length of the bed of carbon nanotubes, thereby causingcurrent to flow through the metallic carbon nanotubes.
 17. The method ofclaim 12, wherein the carbon nanotubes of the bed are arrangedsubstantially in a single monolayer or partial monolayer.
 18. The methodof claim 12, wherein the polymer mask comprises a self-immolativepolymer.
 19. The method of claim 12, further comprising, afterselectively etching the metallic carbon nanotubes, removing the secondportion of the polymer mask from the bed of carbon nanotubes such thatsubstantially no polymer mask remains on the bed of carbon nanotubes.20. The method of claim 19, wherein removing the second portion of thepolymer mask from the bed of carbon nanotubes comprises depolymerizingthe polymer mask by heating the polymer mask.
 21. The method of claim12, wherein, after depolymerizing the first portion of the polymer mask,the polymer mask defines a plurality of troughs exposing the metalliccarbon nanotubes, wherein each trough defines a width between 0.5 nm and10 nm.
 22. A method for carbon nanotube purification, comprising:coating a bed of carbon nanotubes with a polymer mask, the bedcomprising a first population of carbon nanotubes and a secondpopulation of carbon nanotubes, wherein the polymer mask comprises: afirst portion coating the first population and not the secondpopulation; and a second portion coating the second population and notthe first population; selectively depolymerizing the first portion ofthe polymer mask and not the second portion of the polymer mask,comprising selectively heating the first population more than the secondpopulation, thereby heating the first portion of the polymer mask; andafter depolymerizing the first portion of the polymer mask, selectivelyetching the first population of carbon nanotubes and not the secondpopulation of carbon nanotubes, wherein the second portion of thepolymer mask protects the second population during etching; and afterselectively etching the first population of carbon nanotubes, removingthe second portion of the polymer mask from the bed of carbon nanotubes.